1. Technical Field
The invention relates to the field of medical tubing, and more particularly medical tubing suited for catheters and other elongated medical devices.
2. Related Devices and Methods
Vascular disease is a leading cause of premature mortality in developed nations. Treatment of vascular disease may include implantation of tissue supporting stents or prosthetic vasculature, e.g., grafts, stent-grafts, etc., which are delivered through the vasculature at a reduced dimension for ease of navigation in, and reduced chance of injury to, the tortuous vasculature from entry point to the diseased location. Such treatment may include angioplasty performed by expanding a compliant, semi-compliant, or non-compliant balloon in narrowed vessels, which may be narrowed for one or more reasons including the presence of a calcified lesion. These treatments are delivered using a catheter designed to be advanced through the vasculature from a point of entry to the site where the treatment is needed. Such catheters typically include an elongated shaft with a distal end, which is the end furthest from the medical professional advancing the catheter. Such shafts may have variable designs as best suited to deliver the desired treatment from the point of entry to the vasculature to the intended implantation site. Some devices include additional features such as tapered tips on the distal ends of the elongated shafts, sheaths or outer members disposed about much of the length of the elongated shaft and about the vascular implant, and various features on the proximal end, that is, the end closest to the medical professional to perform varied functions, e.g., release of dye or other visualization agent, valved access to a lumen running through the elongated shaft for inserting a guide wire, sealed attachment of a pressurized fluid to inflate balloons at the distal end, or other mechanisms involved in the controlled delivery of the treatment to its intended site. This disclosure describes medical tubing useful as one or more components of such medical devices, including catheters. Unless otherwise stated, the other structural variations in the construction of the catheters of which the present invention is a component are not germane to the present invention.
In many cases, the catheter is an elongated device, which includes one or more elongated tubular members. It may have an elongated balloon at or near its distal end. It may have a tubular member having a lumen through which the catheter follows the path of a previously placed guide wire in the vasculature. Such a tubular member is often referred to as a guide wire tube. The catheter may additionally have another tubular member having a proximal end and a distal end and a lumen therethrough, and surrounding a length of the guide wire tube, but having a different inner diameter than the outer diameter of the guide wire tube such that a roughly annular lumen remains. Such a tubular member is sometimes referred to as an outer member, and the guide wire tube may then alternately be referred to as an inner member. The guide wire tube or inner member may extend distally of the distal end of the outer member. An inflatable member may sealingly surround at its proximal end the external surface of the distal end of the outer member and the external surface of the distal end of the inner member at its (the inflatable member's) distal end. Such sealing may be accomplished by thermal bonding of bondable materials or through the use of a tie layer or adhesive between concentric surfaces. Such a fluid-tight attachment of the inflatable member to the inner and outer member places the interior of the inflatable member in fluid communication with the substantially annular lumen formed between the inner member and the outer member. A typically much shorter length (than either the inner or outer member's length) and separate tubular member having a proximal end and a distal end and a lumen therethrough may have a tapered outer surface and may be attached to the distal end of the elongated shaft, either through a butt joint with the annular surface of the distal end of the inner member or through a lap joint with the outer cylindrical surface of the distal end of the inner member. Alternate attachment constructions of distal tips to the distal end of the catheter are known in the art. For example, see U.S. Pat. Pub. No. 2003/0114794 A1 and U.S. Pat. No. 5,964,778 to Fugoso et al., which are expressly incorporated by reference to the extent not contradictory to the remainder of the present specification.
Balloon catheters used to perform angioplasty in coronary arteries are sometimes referred to as percutaneous transluminal coronary angioplasty (“PTCA”) catheters. These catheters are inserted at a point of entry in a peripheral artery, e.g., a radial or femoral artery, and advanced “upstream” through the arteries to the aorta and to the ostium of the specific coronary artery in need of treatment. The force to advance the catheter is applied external to the patient on an external portion of the catheter, and the force is transferred to the distal tip of the catheter as a result of the column strength or axial stiffness of the catheter. The stiffer the catheter, the easier to advance the distal tip and any therapeutic device or beneficial agent to the desired site. The quality of being able to push a proximal portion of the elongated shaft and have the distal tip move along an artery the same distance is commonly referred to as “pushability.” However, a vascular balloon catheter also has the need to track closely to a guidewire that is resident in the arterial network and follows the tortuous path of the arteries. That quality is commonly called “trackability.” Axial flexibility is needed to have high trackability, and therefore closely track a tortuously curved guidewire. Yet another desirable characteristic of a vascular balloon catheter designed to traverse a calcified lesion or other stenotic area in a coronary artery or other blood vessel is called “crossability,” or, i.e., the catheter's ability to have the distal tip cross the stenosis by the medical professional pushing on the proximal end or some external (to the patient) portion of the catheter.
PTCA catheters that advance through the arteries by sliding over a previously placed guide wire have two commercially available variations, in terms of whether the entire (or almost the entire) length of the catheter slides over the guide wire, or whether only a shorter length of the catheter, at its distal end, slides over the guide wire. Another way to express the difference between the two constructions is whether the guide wire lumen runs the entire length of the catheter and has a proximal port in the proximal end (or in one of the two ports present in the commonly used Y connectors) of the catheter, or whether the proximal port of the guide wire lumen is closer to the distal end of the catheter than the proximal end, and generally between 9 and 20 centimeters proximal of the distal end, or even closer, such as at the proximal end of the inflatable member. The case where the guide wire runs through the entire catheter, or i.e., the guide wire lumen runs through the entire length of the catheter is commonly called an “over-the-wire” (“OTW”) catheter. The case where the guide wire runs through a shorter length of the catheter near the distal end, or i.e., the guide wire lumen has a proximal port closer to the distal end than the proximal end is commonly called a “rapid-exchange” or “Rx” catheter. Another known term for the second case is single-operator-exchange (“SOE”) catheter.
It is common in either type of PTCA catheter, OTW or Rx, to have a proximal portion with greater axial stiffness and a distal portion with a lower relative axial stiffness. This may be accomplished by a tapered support mandrel extending from the proximal end to a point proximal of the distal end, or selection of tubular segments with gradually decreasing column strength. In Rx catheters, it is common to use a relatively stiff proximal tubular member (shaft), such as a metal hypotube, and to attach a relatively less stiff (more flexible) polymer shaft to the distal segment of the hypotube. There may be additional metal or polymer members (e.g., stiffening wires, support tubes, reinforcing tubes, support mandrels, coils, patterned perforations in or “skived” sections of the hypotube, etc.) in Rx designs to provide a gradual transition in axial stiffness to the catheter as a whole from a relatively constant high stiffness proximal shaft to at least the proximal port of the guide wire lumen, which is present along side, or in the side of, the outer member. Without such a transition, the abrupt change in column strength between the end of the hypotube and the proximal port of the guidewire results in a tendency of the catheter to close the inflation lumen when the catheter buckles or kinks in that section of the catheter as a result of the axial loading, which closure of the inflation lumen is undesirable. The distal shaft of an Rx PTCA catheter, as with the distal portion of an OTW PTCA catheter, is generally more flexible, as this is the part that follows the aortic arch and is advanced through small coronary arteries, which tend to be rather tortuous in addition to having a much smaller diameter.
Two of the factors that influence the current PTCA catheter crossability are flexibility and pushability of the distal shaft of an Rx catheter or of the distal portion of an OTW catheter. For example, the more flexible a distal shaft or distal portion is, the higher the crossability of a catheter given the same axial stiffness, or i.e., pushability. Mechanically, however, a shaft that is flexible enough to track through tortuous vessels may not have the column strength to transmit sufficient force axially, while a stiff shaft can transmit greater force axially, but not be flexible enough track along the guidewire through a tortuous vessel. The trade off with increasing flexibility is the reduction of pushability and vice versa.
One commercially available design of the catheter inner body (yet another name for the inner member or guide wire tube), outer body (another name for the outer member), or tip includes a multilayer co-extrusion of different materials on top of each other, e.g., forming radial layers. There is a limitation to this multiple single-material-layer co-extrusion design in that the bending stiffness is uniform in 360 degrees.
The catheter tip is the first component of the catheter to encounter vessel tortuosity, vessel narrowing (stenosis), and calcified stenotic lesions. It is beneficial that the catheter tip is bendable to conform to smaller radius turns of the guide wire in the vasculature after the guide wire has been advanced from the point of entry to, and usually slightly past, the desired site of treatment. A stiff tip that does not conform to the guide wire can displace the guide wire position and may encounter difficulty in navigating through a vessel that has a sharp or abrupt turn. A tip made entirely of flexible material may also not perform as well as desired by “fishmouthing,” a term used to describe when the normally circular cross section of the catheter becomes oval, which increases one dimension and decreases the dimension in the orthogonal direction. That decreased dimension may result in the catheter gripping the guide wire as the dimension approaches the outer diameter of the guide wire. A tip made entirely of flexible material may also fold back on itself, i.e., buckle or bunch up, or alteratively, it may flare, or even evert, which is also not desirable. Thus, it is desirable to have a tip construction that (1) can track gently over the guide wire without displacing the position of the guide wire, (2) minimize “fishmouthing”, and (3) be able to push through stenotic lesions or between the stent struts without one or more of buckling, folding, flaring, or everting.